1. Field of the Invention
This invention relates to a cold sealable emulsion polymer cohesive composition which is low in surface tack, and to cold sealable articles, particularly packaging film treated with the same.
2. Background of the Related Art
In the past, in the preparation of films useful for packaging purposes, heat sealable coatings, such as acrylic coatings, were coated on one side of the film substrate and another heat sealable coating, such as polyvinylidene chloride (PVDC) was coated on the other side. The acrylic coated side was generally the outside of the web, the side in direct contact with the hot sealer surfaces, where good hot slip and jaw release characteristics are required. The PVDC coating was usually on the inside of the web and provided the high seal strength, good hot tack characteristics and barrier properties required for such packaging. These heat sealable coatings have glass transition temperatures ("Tg"s) which are higher than room temperature. Such a coated film is disclosed in U.S. Pat. No. 4,403,464.
Similarly, U.S. Pat. No. 4,456,741 discloses heat sealable terpolymer compositions useful as pressure-sensitive adhesives for application with, for example, backing materials including paper, polyester film and foamed polymers. The terpolymer heat sealable pressure-sensitive adhesive composition comprises butyl acrylate, N-vinyl-2-pyrrolidinone and styrene. Other heat sealable coatings are disclosed in U.S. Pat. No. 3,696,082; and East German Patent No. DD-146,604.
In packaging products which are sensitive to heat, such as candies, chocolates, ice cream and the like, in plastic film or paper packages, the use of heated elements must be avoided in order to prevent melting of the products. Therefore, the use of heat sealable coatings to seal the packages has presented serious difficulties in packaging the product, often requiring the isolation of the product from the heated elements. Accordingly, cold sealable pressure-sensitive adhesives were developed which did not require the use of a heated element to seal the packages. Unfortunately, these adhesives had high surface tack characteristics which made them adhere to uncoated surfaces of the packaging film, making these adhesives difficult to use due to the resulting blocking (i.e. sticking) of the film.
Findley 207-939, a polyisoprene adhesive manufactured by Findley Adhesive, Inc., is a cold sealable pressure-sensitive adhesive coating which exhibits good crimp seal strength on oriented polypropylene film and has a Tg of -1.6.degree. C. This adhesive has a high surface tack which often results in blocking of the packaging film.
Another such pressure-sensitive adhesive composition is disclosed in U.S. Pat. No. 4,500,683. The pressure sensitive adhesives of this patent contain as a polymer component an addition polymerization polymer of an acrylate or methacrylate polymer and one or more ethylenically unsaturated monomers capable of forming homo- or copolymers having a glass transition temperature of at least 273.degree. k. This composition forms a viscous adhesive composition which is tacky at room temperature, thus presenting the blocking problems when used on packaging films.
Thus, the related art has disclosed heat sealable coatings and pressure-sensitive, tacky, cold sealable adhesives useful in the packaging art. The related art has not, however, disclosed or provided a cold sealable, pressure-sensitive cohesive, rather than adhesive formulation which similarly to the above described heat sealable, pressure-sensitive coatings, has low surface tack and is cohesive only when placed under pressure in contact with other cold sealable cohesive coated surfaces.
Accordingly, it is an object of the present invention to provide a coal sealable, emulsion polymerized terpolymer cohesive formulation which is cohesive to similarly coated film or paper substrates, rather than being tacky and adherent to non-coated surfaces.
Another object of the present invention is to provide a cold sealable cohesive formulation having the unique improved combination of properties, e.g., low surface tack, good coating uniformity, high seal strength and wetting ability, on substrates such as plastic and paper film.
Still a further object of the invention is to provide a cold sealable, emulsion polymer cohesive formulation which, when coated on a film or paper substrate, imparts high cold seal strength, i.e., bonding under only pressure, even at low coating weight and also improved surface properties, e.g., low surface tack and excellent coating uniformity.
Still another object of the present invention is to provide improved packaging film or paper coated with a cold sealable cohesive formulation having a smooth, non-tacky surface which will not block at room temperature.
A further object of the invention is to provide improved coated packaging film having high cold seal strength.